Extract first N characters in a string in C++

This post will discuss how to extract the first few characters of a string in C++.

1. Using `std::string::substr`

The standard solution to extract the first n characters of a string is using the std::string::substr function. It takes the index of the first character and the number of characters to be extracted.

#include <iostream>

#include <string>

std::string findSubstr(std::string const &str, int n) {

if (str.length() < n) {

return str;

}

return str.substr(0, n);

}

int main()

{

std::string str = "C++20";

int n = 3;

std::string substr = findSubstr(str, n);

std::cout << substr << ' '; // C++

return 0;

}

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2. Using `std::string::resize`

Alternatively, we can use the std::string::resize function to resize a string to the specified length. To avoid modifications to the original string, invoke resize on a copy of the string, as shown below:

#include <iostream>

#include <string>

std::string findSubstr(std::string str, int n) { // no-ref, no-const

str.resize(n);

return str;

}

int main()

{

std::string str = "C++20";

int n = 3;

std::string substr = findSubstr(str, n);

std::cout << substr << std::endl; // C++

return 0;

}

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3. Using `std::copy_n`

We can also use the std::copy_n algorithm from header <algorithm> to extract the first n characters from a string. It can be used as follows to copy the first n characters:

#include <iostream>

#include <string>

#include <algorithm>

#include <iterator>

std::string findSubstr(std::string const &str, int n) {

std::string s;

std::copy_n(str.begin(), n, std::back_inserter(s));

return s;

}

int main()

{

std::string str = "C++20";

int n = 3;

std::string substr = findSubstr(str, n);

std::cout << substr << std::endl; // C++

return 0;

}

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That’s all about extracting the first few characters of a string in C++.

Extract first N characters in a string in C++

1. Using std::string::substr

2. Using std::string::resize

3. Using std::copy_n

Share this:

1. Using `std::string::substr`

2. Using `std::string::resize`

3. Using `std::copy_n`